1. Field of Invention
The present invention relates in particular to the area of devices or systems whose optical operation is modified by changing the optical properties of the medium of which they are formed and that are intended in particular for limiting the luminous intensity transmitted by the system. In particular, it relates to a broadband photoactivated device for limiting luminous flux and can be used, for example, to protect an optronic sensor or the eye against aggressions from intense radiation, such as that emitted by a pulsed laser.
2. Description of Related Art
The development of intense light sources, possibly pulsed, wavelength-tunable light sources, has brought about the need to protect optical systems such, as the human eye or detectors operating in visible or infrared ranges that could be subjected to radiation from such a source.
This protection consists of placing an optical limitation device between the system to be protected and the source. This device should preferably have a good transmission coefficient in the case of incident radiation whose intensity is insufficient to damage the detector or any observation system and react immediately to high-luminous-flux incident radiation that could damage the detector. The protective response results in a decreased transmission coefficient so that the luminous flux emerging from the optical limitation device is insufficient to damage the detector or observation system.
Of all the devices able to attenuate radiation and having the aforesaid optical limitation devices, four types may be cited.
The first type of device uses suspended carbon particles that may be in the form of India ink highly diluted in a liquid. This type of device transmits ambient light perfectly but its optical properties change when it is subjected to very-high-intensity radiation, such as that emitted by a pulsed laser, and this change tends to decrease transmission. The luminous flux leaving the device is hence less than that entering the device.
This type of device gives good results in terms of protection against luminous fluxes with wavelengths from the visible to the near infrared, but its implementation raises many problems, such as that of keeping the carbon particles in suspension.
The second type of device relates to Christiansen filters made of small pieces of glass immersed in a liquid whose refractive index is equal to that of the glass but which varies under the influence of high-intensity radiation. Thus, this device is transparent to low- and medium-intensity radiation and becomes a diffuser when high-intensity radiation passes through it. The luminous flux at the output of the device is hence less than that at the input.
The third type relates to crystalline colloidal filters which are particles suspended in a liquid that have the particular property of organizing into a crystal-like structure. The liquid is chosen so that its refractive index is equal to that of the particles but varies under the influence of high-intensity radiation.
Thus, this device is transparent to low- and medium-intensity radiation and becomes diffracting when high-intensity radiation passes through it.
Christiansen filters and colloidal filters have one major drawback. That is, under some circumstances, they are spectrally selective, meaning that they have the aforesaid properties only for one wavelength or for a restricted wavelength range. Thus, this type of filter is, for example, unsuitable for protecting an optronic device against the radiation emitted by a pulsed wavelength-tunable laser system.
The fourth type is described in patent EP538864 and relates to the use of fullerenes, such as C60 to limit the intensity of visible radiation by the effect of reverse saturable absorption. However, beyond visible radiation, for example, at a wavelength of 800 nm, this C60 is no longer effective and, to obtain good efficiency over a wavelength range not confined to visible radiation, it is necessary to use it in combination with higher-order fullerenes, such as C70 or C80. However, using such a complex combination of materials is difficult because it requires toxic solvents to be used and hence restrictive measures to be taken to ensure manufacturing safety.
The goal of the invention is to overcome these drawbacks by providing a device able to transmit low- or medium-intensity incident radiation, namely radiation unlikely to damage an optical system, such as a detector or the human eye, and immediately to attenuate incident radiation whose intensity could damage the optical system, and do so over a large wavelength range.
The solution is a photoactivated device that limits the luminous flux of radiation, constituted by a liquid or solid medium containing nanotubes, for example, nanotubes made of carbon or boron, tungsten, silicon, or composite nanostructures, such as BN, BC3, BC2N, CN(C3N4), or MoS2. According to one particular characteristic, the medium has single-wall and/or multi-wall carbon nanotubes. These carbon nanotubes may be of a known type and can be made, for example, with an electric arc, by laser ablation, with a solar oven, or chemically.
According to another particular characteristic, the medium may include, carbon nanotubes doped or functionalized with chemical entities chosen from iodine, krypton, argon, the OH group, metals or semiconductor materials, or materials with optically nonlinear properties. According to one particular characteristic, the carbon nanotubes may be suspended in a liquid or incorporated into a gel or into a solid matrix.
According to an additional characteristic, the solid matrix may include at least one polymer, which may or may not be mesomorphic, or by a solgel, possibly in combination with a liquid crystal.
According to a particular characteristic, a generator for generating a variable-frequency electrical field and/or a magnetic field and/or an induction may be associated with the medium for orienting the nanotubes. According to another characteristic, the generator for generating an induction may be associated with the matrix.
The invention also relates to a detector of the type having at least one objective and one radiation-sensitive element Rs and having a device for limiting a luminous flux according to any of the above-listed characteristics.
The invention also relates to a photoactivated method for broadband limitation of the luminous flux of a radiation, including placing a medium that includes a liquid or solid medium containing nanotubes, for example, nanotubes of carbon or boron, tungsten, silicon, or composite nanostructures, such as BN, BC3, BC2N, CN(C3N4), or MoS2, in the path of this radiation.
A device according to the invention enables the incident luminous flux to be attenuated starting from a certain threshold. When the luminous flux is low or moderate, which is the case with ambient light, the medium constituted by the medium containing the nanotubes appears as a transparent medium. The incident luminous flux Ri is then practically unattenuated because about 80% or even more of this flux passes through it.
When the device receives a high level of luminous flux, the luminous flux leaving it has a flux limited to a value compatible with the normal operation of an optronic system. The incident luminous flux activates protection when this threshold is exceeded. The medium is thus self-activated by the flux. When the device no longer receives a high level of luminous flux, it appears as a transparent system once more.
The invention can, in particular, be applied to optronic devices, such as, for example, a TV camera or missile seeker.
A device according to the invention is broadband. Thus, it may operate, in particular, in the visible range and in the near infrared range.